Experimental Sorption Isotherms

It is relatively easy to obtain equilibrium sorption data for a single sorbate in the laboratory. A small amount of the sorbent is brought into contact with a solution containing the sorbate of interest. The conditions of the sorption system, particularly pH, must be carefully controlled at the required values over the entire period of contact until the sorption equilibrium is reached. This may take a few hours or much longer, depending on the size of the sorbent particles and the time it takes until they attain sorption equilibrium.

A simple preliminary sorption kinetics test will establish the exposure time necessary for the given sorbent particles to reach the equilibrium state. The following procedure provides an example for obtaining the experimental sorption equilibrium data points for the isotherm:

1. Prepare the sorbate in solution at the highest concentration of interest.
2. Prepare dilutions covering the entire concentration range, from 0 (blank) to the maximum.
3. Adjust the conditions, e.g., pH, ionic strength, etc.
4. Determine the sorbate initial concentrations (C_i ) in all the liquid samples.
5. Distribute the samples into containers of appropriate volumes (30-150 mL of liquid) such as flasks or test tubes; prepare samples in duplicate, triplicate or as required.
6. Accurately weigh each quantity of the biosorbent solids to be used in the tests and record the weights (S, mg). It may help to be able to roughly estimate the anticipated sorption uptake so that there is an easily detectable final sorbate concentration in each sample solution at equilibrium. If too much sorbent is added, there may be virtually no sorbate left in the solution, precluding a reliable analysis. Varying the initial concentration could cause the sorbent weight to fluctuate, which has to be precisely known for each sample. Metal depletion in the solution must be avoided because it renders such samples useless.
7. Add the sorbent solids into each sample solution and provide rather gentle mixing over the contact period.
8. Make sure the conditions (especially pH) are controlled at constant values during the contact period. Use an appropriate acid or base for this; do not dilute the sorption system by adding excessive volume.
9. At the end of the contact period, separate the solids from the liquid by decantation, filtration, centrifugation, etc.
10. Analyze the liquid portion to determine the residual final sorbate concentration (C_f).
11. Calculate the sorbate uptake: q = V [L] (C_i - C_f) [mg/L] / S [g]. Note that q could also be determined directly by analyzing the separated solids and thus closing the material balance on the sorbate in the system. However, this usually presents analytical difficulties (digestion-liquefaction of solids, and/or very sophisticated analytical methods may be required).
12. Plot the sorption isotherm q vs. (C_f).

Comparison of Sorption Performance

The performance of sorbing materials needs to be evaluated and often compared. The simplest situation is when there is only one sorbate species in the system, in which case it is best to base the single-sorbate sorption performance on a complete single-sorbate sorption isotherm curve.

To fairly compare two or more sorbents, the comparison must be done under uniform conditions. These may be restricted by the environmental factors under which sorption may have to take place (pH, temperature, ionic strength, etc.), which may not necessarily be easily or widely adjustable. In particular, it is important to compare sorption performance under the same pH conditions, since isotherms can vary with pH.

The performance of the sorbent is usually gauged by its uptake (q). Sorbents can be compared based on their respective maximum uptake values (q_max), which can be calculated by fitting the Langmuir isotherm model to the actual experimental data (if it fits). This approach is feasible if q_max reaches a plateau. Some isotherms might not exhibit the asymptotic plateau represented by the Langmuir equation.

In general, one is looking for a "good" sorbent with a high sorption uptake capacity (q_max). Surface area in biosorption is not particularly important.